1. Field of the Invention
The present invention relates to a pulse wave measuring apparatus, particularly a pulse wave measuring apparatus of high measurement accuracy at low cost.
2. Description of the Background Art
Conventional pulse wave measuring apparatuses include the type measuring the pulse wave with a pressure sensor set right above an artery. It is extremely difficult to accurately position the pressure sensor right above an artery in such pulse wave measuring apparatuses. Technique of high level was required for the positioning of such pulse wave measuring apparatuses. There was also the problem that the reproducibility of measurement is poor since the positioning reproducibility is not good.
There is known a sphygmograph apparatus employing tonometry, directed to overcome the problem set forth above.
The mechanism of tonometry will be described with reference to FIG. 16. Referring to FIG. 16, the artery is pressed superficial of the body with a flat plate, whereby the artery is deformed to a leveled form. At the region right above the flat artery, the effect of blood vessel tension indicated by the dotted arrow in FIG. 16 on the pressure in the blood vessel is smallest since the blood vessel tension is balanced between the left and right sides. This means that the pressure measured by a sensor element smaller in size than the flat pressed region right above the leveled artery is consistent with the intra-arterial pressure. Thus, the intra-arterial waveform can be measured superficial of the body.
One type of a conventional pulse wave measuring apparatus employing tonometry has a plurality of small sensor elements aligned, as the pressure sensor positioned right above an artery to depress the contacting region for measurement of a pulse wave. The pulse wave is measured by the sensor element located right above the artery. Such a blood pressure monitor employing tonometry is disclosed in, for example, Japanese Patent No. 2776961.
Since the pulse wave measuring apparatus employing tonometry has a plurality of small sensor elements aligned, the possibility of any one of the sensor elements being located right above the artery is high. Therefore, positioning of the apparatus is facilitated. The positioning of a plurality of aligned sensor elements is disclosed in, for example, Japanese Patent Laying-Open No. 2002-320594, filed by the applicant of the present application prior to the filing of the present invention.
The pulse wave measuring apparatus employing tonometry set forth above must have a plurality of sensor elements as small as approximately 0.2-0.3 mm in width, for example, aligned. Accordingly, critical requirements such as high sensitivity and microfabrication must be satisfied. This necessitates the usage of a semiconductor silicon MEMS (Micro Electro Mechanical Systems) pressure sensor, resulting in an expensive sensor. Further, the complexity of electronic circuitry receiving the sensor signals will be increased since signals from many sensor elements are to be processed. There was a problem that the cost is increased.
If the aforementioned small sensor elements are not used in the pulse wave measuring apparatus directed to overcome the problem set forth above, the leveled region of an artery will become smaller than the width of the sensor element, leading to the problem of measurement error. This problem will be described in detail with reference to FIG. 17. FIG. 17 shows the AI (Augmentation Index), calculated based on the pulse waves measured by respective sensor elements of small size (0.2 mm in width) aligned on an artery. The AI is a parameter significantly affected by distortion in a sensor signal. This AI will be described in detail in the section of the embodiments of the present invention.
It is appreciated from FIG. 17 that the increase of an AI value representing the degree of sensor signal distortion becomes greater as a function of distance from the leveled region. This means that the degree of sensor signal distortion becomes larger. As shown in FIG. 16, this arises from that fact that, as located farther from the leveled region, the effect of the resultant force of the blood vessel tension on the pressure in the blood vessel will become greater due to the generation of blood vessel tension in a direction other than the direction parallel to the leveled region.
A larger width of the sensor element consequently allows a larger range of detection, leading to the possibility of a region other than the leveled region being included. As a result, the possibility of a region of high sensor signal distortion as shown in FIG. 17 being included in the range of detection will be increased. Thus, the possibility of measurement error caused by distortion in a sensor signal from a sensor element is high. Such a problem is similarly encountered, not only in the case where the sensor element is large in width, but also in the case where the pressurization force of the sensor element is insufficient, or when the sensor element is pressed back in response to a high intra-arterial pressure.